Plasmas are often used to activate gases placing them in an excited state so that they have an enhanced reactivity. In some cases, the gases are excited to produce dissociated gases containing ions, free radicals, atoms and molecules. Dissociated gases are used for numerous industrial and scientific applications including processing materials such as semiconductor work pieces (e.g., wafers), powders, and other gases. The parameters of the dissociated gas and the conditions of the exposure of the dissociated gas to the material being processed vary widely depending on the applications.
Plasma reactors for processing semiconductor wafers may form a plasma within a chamber containing the wafer, or they may receive excited gases produced by a reactive gas generator located upstream of the chamber. The preferred location of plasma generation relative to the wafer location depends on the process.
In some processes the plasma affects the wafer through direct contact between the plasma and the wafer. Direct contact may be desirable because a wafer in contact with a plasma generally has increased chemical reactivity due to the presence of electrons and ions in the plasma. Further, when plasma is in contact with the wafer, it is possible to control the energy and direction of ions at the wafer surface by applying a bias voltage to the wafer. Such arrangements are used in, for example, plasma-enhanced chemical vapor deposition or directional etching applications.
In other processes, plasma is generated away from the wafer, and then excited gases from the plasma come into contact with the wafer. For semiconductor processes in which the wafer is sensitive to electric charges in a plasma, susceptible to ultraviolet energy (UV) damage generated by the plasma, or which require high chemical selectivity, exposing the wafer to the plasma can be undesirable. In some situations, the wafer and the plasma chamber surfaces can be damaged by exposure to chemically corrosive plasmas, which may create chemical contamination and particle generation, shorten the product life and increase cost of ownership. Accordingly, remote plasma sources are sometimes used to reduce wafer and chamber damage by generating plasma outside the process chamber and then delivering activated gases produced by the plasma to the processing chamber for processing the wafer.
Reactive gas generators generate plasmas by, for example, applying an electric potential of sufficient magnitude to a plasma gas (e.g., O2 N2, Ar, NF3, F2, H2 and He), or a mixture of gases, to ionize at least a portion of the gas. Plasmas can be generated in various ways, including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharge plasmas are achieved by applying a potential between two electrodes in a plasma gas. RF discharge plasmas are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Microwave discharge plasmas are achieved by directly coupling microwave energy through a microwave-passing window into a discharge chamber containing a plasma gas. Plasmas are typically contained within chambers having chamber walls that are composed of metallic materials such as aluminum, or dielectric materials such as quartz, sapphire, yttrium oxide, a zirconium oxide, and/or an aluminum nitride. The plasma chamber can include a metal vessel having walls coated with a dielectric material.
In some applications a plasma or an excited gas may not be compatible with the reactive gas generator and/or the semiconductor processing system. For example, during semiconductor manufacturing, ions or atoms of fluorine or fluorocarbons may be used for etching or removing silicon or silicon oxides from surfaces of semiconductor wafers or for cleaning process chambers. Because the fluorine ions are chemically reactive and corrosive to process chamber materials, remote plasma sources have been used to generate atomic fluorine for these processes to avoid damaging the process chamber. Although the use of a remote plasma source reduces corrosion/erosion in the process chamber, some corrosion/erosion still occurs in the remote plasma source.
In another example, atomic oxygen is used to remove photoresist from a semiconductor wafer by converting the photoresist in to volatile CO2 and H2O byproducts. Atomic oxygen is typically produced by dissociating O2 (or a gas containing oxygen) with a plasma in a plasma chamber of a reactive gas generator. Atomic fluorine is often used in conjunction with atomic oxygen because the atomic fluorine accelerates the photoresist removal process. Fluorine is generated by, for example, dissociating NF3 or CF4 with the plasma in the plasma chamber. Fluorine, however, is highly corrosive and can adversely react with various materials used for chambers, such as aluminum.
A problem that plagues many different types of equipment used in semiconductor fabrication, including plasma chambers, is copper contamination. Because copper is a “fast diffuser” (i.e., has a higher diffusion rate in typical semiconductor materials than many other elements), introducing very small amounts of copper in semiconductor fabrication equipment can cause failure of semiconductor devices. Further, small amounts of copper can be easily transferred from one piece of equipment to another, thereby spreading and contaminating semiconductor fabrication equipment in a fabrication line.
A need therefore exists for improved protective coatings that are less susceptible to the corrosive affects of excited gases located in a plasma chamber without contributing to the problem of copper contamination.